This invention relates to process control and, more particularly, to improved methods and apparatus for process control utilizing optimum setpoint determination.
Process control refers to a methodology for controlling the operational parameters of a process by monitoring one or more of its characteristics over time. It is used to ensure that the quality and efficiency of the process do not vary substantially during a single run or over the course of several runs. While process control is typically employed in the manufacturing industry, it also has application in the service industry.
A process control unit, or controller, compares values of a process characteristic, or "controlled variable," with a target value to determine whether the process is operating within acceptable bounds. For example, in a biscuit baking process, a controller compares the moisture of the baked product with a desired moisture content value to determine whether batches of biscuits are properly baked.
If the controller determines that the controlled variable is outside normal bounds, it alerts the operator to the need for corrective action. Alternatively, the controller can initiate corrective action itself by adjusting a parameter, or "manipulated variable," of the process. In the biscuit baking example, if the sample is too moist, the controller can signal an increase in the baking time.
One technique of process control, statistical process control, or SPC, is based on the assumption that, when a process is in control, measured values of the controlled variable will be normally distributed about the target and lie within established limits. When that variable falls significantly outside the expected pattern, the controller takes the corrective action.
A limitation of many prior controllers is that they do not account for process specification limits. These limits, typically expressed in percentages, define how much process output can fall outside a specified limit on one or both sides of a target. For example, in biscuit production, the specification may establish a goal of 15% moisture content, with a limitation that not more than 5% of a batch have a moisture content above 22% or below 10%.
In view of the foregoing, an object of this invention is to provide improved process control apparatus and methods. More particularly, an object of the invention is to provide a method and apparatus for process control with improved setpoint determination.
A further object of the invention is to provide a method and apparatus for optimum setpoint control that utilizes process specification limits in defining an efficient target for process output.